Macular degeneration affects approximately 1.7 million individuals in the U.S. and is the most common cause of acquired visual impairment in those over the age of 65. Stargardt disease (STGD; McKusick Mendelian Inheritance (MIM) #248200) is arguably the most common hereditary recessive macular dystrophy and is characterized by juvenile to young adult onset, central visual impairment, progressive bilateral atrophy of the macular retinal pigment epithelium (RPE) and neuroepithelium, and the frequent appearance of orange-yellow flecks distributed around the macula and/or the midretinal periphery (Stargardt, 1909; Anderson et al., 1995). A clinically similar retinal disorder (Fundus Flavimaculatus, FFM, Franceschetti, 1963) often displays later age of onset and slower progression (Fishman, 1976; Noble and Carr, 1979). From linkage analysis, it has been concluded that STGD and FFM are most likely allelic autosomal recessive disorders with slightly different clinical manifestations caused by mutation(s) of a gene at chromosome 1p13–p21 (Gerber et al., 1995; Anderson et al., 1995). The STGD gene has been localized to a 4 cM region flanked by the recombinant markers D1S435 and D1S236 and a complete yeast artificial chromosome (YAC) contig of the region has been constructed (Anderson et al., 1995). Recently, the location of the STGD/FFM locus on human chromosome 1p has been refined to a 2 cM interval between polymorphic markers D1S406 and D1S236 by genetic linkage analysis in an independent set of STGD families (Hoyng et al., 1996). Autosomal dominant disorders with somewhat similar clinical phenotypes to STGD, identified in single large North American pedigrees, have been mapped to chromosome 13q34 (STGD2; MIM #153900; Zhang et al., 1994) and to chromosome 6q11–q14 (STGD3; MIM #600110; Stone et al., 1994), although these conditions are not characterized by the pathognomonic dark choroid observed by fluorescein angiography (Gass, 1987).
Members of the superfamily of mammalian ATP binding cassette (ABC) transporters are being considered as possible candidates for human disease phenotypes. The ABC superfamily includes genes whose products are transmembrane proteins involved in energy-dependent transport of a wide spectrum of substrates across membranes (Childs and Ling, 1994; Dean and Allikmets, 1995). Many disease-causing members of this superfamily result in defects in the transport of specific substrates (CFTR, Riordan et al., 1989; ALD, Mosser et al., 1993; SUR, Thomas et al., 1995; PMP70, Shimozawa et al., 1992; TAP2, de la Salle et al., 1994). In eukaryotes, ABC genes encode typically four domains that include two conserved ATP-binding domains (ATP) and two domains with multiple transmembrane (TM) segments (Hyde et al. 1990). The ATP-binding domains of ABC genes contain motifs of characteristic conserved residues (Walker A and B motifs) spaced by 90–120 amino acids. Both this conserved spacing and the “Signature” or “C” motifjust upstream of the Walker B site distinguish members of the ABC superfamily from other ATP-binding proteins (Hyde et al., 1990; Michaelis and Berkower, 1995). These features have allowed the isolation of new ABC genes by hybridization, degenerate PCR, and inspection of DNA sequence databases (Allikmets et al., 1993, 1995; Dean et al., 1994; Luciani et al., 1994).
The characterization of twenty-one new members of the ABC superfamily may permit characterization and functions assigned to these genes by determining their map locations and their patterns of expression (Allikmets et al., 1996). That many known ABC genes are involved in inherited human diseases suggests that some of these new loci will also encode proteins mutated in specific genetic disorders. Despite regionally localizing a gene by mapping, the determination of the precise localization and sequence of one gene nonetheless requires choosing the certain gene from about 250 genes, four to about five million base pairs, from within the regionally localized chromosomal site.
While advancements have been made as described above, mutations in retina-specific ABC transporter (ABCR) in patients with recessive macular dystrophy STGD/FFM have not yet been identified to Applicant's knowledge. That ABCR expression is limited to photoreceptors, as determined by the present invention, provides evidence as to why ABCR has not yet been sequenced. Further, the ABC1 subfamily of ABC transporters is not represented by any homolog in yeast (Michaelis and Berkower, 1995), suggesting that these genes evolved to perform specialized functions in multicellular organisms, which also lends support to why the ABCR gene has been difficult to identify. Unlike ABC genes in bacteria, the homologous genes in higher eukaryotes are much less well studied. The fact that prokaryotes contain a large number of ABC genes suggests that many mammalian members of the superfamily remain uncharacterized. The task of studying eukaryote ABC genes is more difficult because of the significantly higher complexity of eukaryotic systems and the apparent difference in function of even highly homologous genes. While ABC proteins are the principal transporters of a number of diverse compounds in bacterial cells, in contrast, eukaryotes have evolved other mechanisms for the transport of many amino acids and sugars. Eukaryotes have other reasons to diversify the role of ABC genes, for example, performing such functions as ion transport, toxin elimination, and secretion of signaling molecules.
Accordingly, there remains a need for the identification of the sequence of the gene, which in mutated forms is associated with retinal and/or macular degenerative diseases, including Stargardt Disease and Fundus Flavimaculatus, for example, in order to provide enhanced diagnoses and improved prognoses and interventional therapies for individuals affected with such diseases.